The invention described herein relates generally to fiducial timing systems for use with interdependent groups of streak cameras, and more particularly to optical fiducial timing systems for use with interdependent groups of streak cameras, especially X-ray streak cameras.
In recent years it has developed that various short duration and rapidly changing phenomena related to plasma physics may be conveniently and accurately recorded by using streak cameras to measure X-ray and/or optical plasma emissions. This has given rise to the construction of large systems of interdependent streak cameras that are designed to measure the salient features, in terms of spectral energy range, intensity, pulse shape and the like, of plasma event experiments. For example, the Nova Laser Facility of the Lawrence Livermore National Laboratory, which presently is a 100-TW/100-kJ-class laser system comprising ten 137-meter-long neodymium glass laser chains that are used to study inertial confinement fusion and other aspects of plasma physics, has such a streak camera diagnostic system surrounding disposed about a target chamber which is 4.4 meters in diameter. Many analogous research facilities, both in the United States and throughout the world, have very extensive, associated streak camera diagnostic systems for the recordation of experimental results.
At these facilities, after an experiment is performed, it would be extremely beneficial to have accurate and absolute reference timing information available for each streak camera data record, so that all the data records could be temporally compared. The reason for this is that the pulsed radiative emissions from a plasma, such as the plasma produced when an inertial confinement fusion target is laser driven, is invariably highly functional of both radiation frequency and emission direction. Thus, for example, if one were given, without timing information, two related streak camera data records that were taken over different radiation frequency ranges, or even over the same radiation frequency range, but in different spatial directions with respect to the emitting source, the records generally could not be temporally cross-compared by simple visual inspection. It is emphasized that this is not a problem of the data records simply having been taken at different sweep rates, as measured, for example, in millimeters per picosecond; it is, rather, a problem of not being able to establish, for each sweep camera data record, an absolute system time for each position along each separate data record.
The problem of how to cross-time the data from a multiplicity of diagnostic instruments that are all recording signals emitted from the same source is not new. The usual solution is to create a single fiducial signal, send it to each recording instrument along separate paths of known time delay, and then record it on each diagnostic instrument along with the normally measured data. Even though recorded together on the same piece of photographic recording film or on the same oscilloscope trace, to give typical examples of recording media, the fiducial signal and the recorded data are usually arranged to be separated in time when recorded, so as not to interfere with one another. By knowing or measuring the time it takes for the single fiducial signal to travel along the various paths to each recording instrument, and the location at which the fiducial signal appears on each data record, one is enabled to associate the fiducial signal appearing on each data record with an individual and known point of time. Thus, the individual data from all the recording instruments may be cross-compared accurately.
In many fiducial timing systems the fiducial signal interacts with each recording instrument by the same physical mechanism as the data signal that the recording instrument is recording. Thus each signal is recorded in sequence at its appropriate time. For example, if a voltage data signal is being recorded by a voltage recording instrument, a fiducial signal in the form of a short voltage pulse is often input to the instrument. On the other hand, if an optical signal is being recorded by an optical recording instrument, then a fiducial signal in the form of a short burst or pulse of light may be directly input to the optical recording instrument at the appropriate time. If this direct approach is not possible or convenient, some other means must be employed whereby the fiducial signal will cause its signature to be impressed on the recording medium at the unique location indicative of fiducial signal time. That is, means must be employed whereby, as the recording instrument writes or otherwise establishes a data record across a data recording medium, the recording instrument will write or record the signature of the fiducial signal on the recording medium at the time when, or as, the fiducial signal arrives at the recording instrument, and at the location on the recording medium where data are then being recorded.
To give a practical example, in many sweep camera data recording instruments a data record is established on a photographic film recording medium when a pulse of electrons is accelerated, focused, and swept across a phosphor screen. Data are recorded in the form of exposure level variations along the exposed path on the film, because the input data signal proportionally controls the instantaneous intensity of the sweeping electrons. In fact, at the times when there is no data signal, the instantaneous intensity of the sweeping electron pulse is zero. Additionally, by employing various slit configurations in the sweep camera, the sweeping electron pulse can be caused to have a transverse structure indicative of the spatial characteristics of the driving source. In these sweep camera data recording instruments, means should be employed to cause a fiducial signal to be recorded on the surface of the film at an appropriate location. In situations where optical streak cameras are recording visible optical data signals, it is relatively easy to arrange for a visible optical fiducial signal to cause a pulsed excursion in intensity in the sweeping electrons. However, in situations where X-ray streak cameras are recording X-ray signals, it is not practical to provide X-ray fiducial signals having sufficient X-ray intensity to be recorded by the cameras. Additionally, in these situations, it is also presently not practical to use non-X-ray fiducial signals to cause X-ray streak cameras to undergo pulsed excursions in sweeping electron pulse intensity. The reasons for this will now be examined.
Reference is made to FIG. 1, prior art, which is a schematic representation of a diagnostic system that includes an X-ray streak camera. A pulsed X-ray source 10 is positioned within the evacuated interior of a housing 12 that is provided with a slit or hole 14 through which a quantity of X-rays 16, emitted from source 10, may pass. An X-ray streak camera 18, the interior of which is evacuated by means not shown, is positioned adjacent to housing 12 so as to receive X-rays 16. X-rays 16 directly impinge upon an X-ray streak camera photocathode 20, within an end surface 21, thereby causing the ejection of electrons from photocathode 20. In actual practice photocathode 20 may have many different configurations and shapes. Photocathode 20 and end surface 21 are shown supported by a photocathode fixture 22 that is, in turn, inserted into a mounting member 24. Mounting member 24 is supported by a photocathode support structure 26 that is attached to an X-ray streak camera housing 28. A schematic cut-away view of photocathode 20, end surface 21, and photocathode fixture 22 is given in FIG. 2, prior art. Returning to FIG. 1, electrons ejected from photocathode 20, by X-rays 16, are accelerated toward an accelerating grid 30, that is caused to be at a positive electrical potential with respect to photocathode 20, by well-known means that are not shown. Grid 30 is supported within mounting member 24. The accelerated electrons are focused into an electron pulse 32 by a focusing electrode 34 that is supported by a focusing electrode support structure 36 that is attached to streak camera housing 28. Electron pulse 32 is continuously swept from a first election path 38 to a final electron path 40 by means of a pair of sweep plates 42, that are held within X-ray streak camera housing 28 by a pair of support rods 44. The means by which an electron pulse is spatially swept by sweep plates are very well-known in the prior art. Since the number of electrons being ejected from photocathode 20 is instantaneously proportional to the intensity of X-ray quantity 16, the intensity of electron pulse 32 is also instantaneously proportional to the intensity of X-ray quantity 16. Electron pulse 32 sweeps across a microchannel plate 46 and a phosphor coated fiber optic faceplate 48, disposed in an image intensifier housing 50, and produces photons that are recorded on film 52, contained in film housing 54.
In operation, X-ray streak camera 18 is triggered, by well-known means that are not shown, so that electron pulse 32 will sweep between paths 38 and 40 as X-ray source 10 emits a short burst or pulse of X-rays, of which X-ray quantity 16 impinges on photocathode 20. When photographic film 52 is developed, an intensity-varying streak of data indicative of the time-varying character of X-ray quantity 16 is revealed. Absolute system timing is totally lacking from this data record. As mentioned above, it is presently not practically feasible to create a fiducial pulse of X-rays having a sufficient intensity to be recorded on photographic film 52.
However, as indicated in FIG. 3, prior art, to whicn attention is now directed, it has been attempted to use an optical, not an X-ray, fiducial signal to drive the data recording mechanism of X-ray streak cameras. FIG. 3 is a sectional side view of a photocathode 60, an end surface 62, and a photocathode fixture 64 of an X-ray streak camera similar to the X-ray streak camera of FIGS. 1 and 2. The prior attempt comprised trying to use an optical pulse of light to produce electrons. An optical light-conducting fiber 66 was inserted through a hole 68 in end surface 62, with hole 68 positioned near photocathode 20, as shown. The attempt was made to send an intense pulse of light along optical fiber 66 having sufficient light intensity to produce a burst of electrons that could be recorded by an X-ray streak camera. The mechanism of electron production in this attempt was to have the light directly interact with optical fiber 66 itself, particularly at impurity sites, and eject electrons therefrom. This attempt substantially failed because bursts or pulses of optical light having an intensity sufficient to produce enough electrons from the end of optical fiber 66 to be sensibly recorded on the film of a streak camera are likewise sufficiently intense to cause serious damage to fiber 66, so that fiber 66 usually cannot be used more than once and has to be replaced after each use. This means that an optical fiducial timing system of this type would have to be totally replaced after each experiment. In diagnostic systems comprising many X-ray streak cameras, this is intolerable.